Methods for determining times and changes of times of transmission, radio communications system and processor

ABSTRACT

The invention relates to a method for determining transmission times of signals transmitted by at least one radio transmitter BTS. In order to enhance the accuracy and reliability of estimates of transmission times or of changes of the transmission time of signals transmitted by radio transmitters BTS, it is proposed that a difference in the time level of estimates of the transmission times based on the measurement results of two different radio receivers LMU A,B is taken into account in a filtering of the estimates for obtaining an accurate value for the transmission times. It is further proposed that the changes of the transmission times estimated at different receivers LMU A,B is averaged for obtaining an accurate information about the changes. The invention equally relates to a corresponding radio communications system and to a processing unit for such a system.

PRIORITY CLAIM

This is a national stage of PCT application No. PCT/EP01/07330, filed onJun. 27, 2001. Priority is claimed on that application.

FIELD OF THE INVENTION

The invention relates to methods for determining times of transmissionand changes in the times of transmissions of signals transmitted by atleast one radio transmitter, to a radio communications system and to aprocessor for a radio communications system.

BACKGROUND OF THE INVENTION

In a radio communications system, it can be of importance to be able todetermine the time at which signals are transmitted by radio transmitterof the system.

The possibility to determine the timing of transmissions of differentradio transmitters can be employed e.g. advantageously to geographicallylocalize a mobile station in a radio communications system. To this end,e.g. first the timing difference between at least two pairs of radiotransmitters is determined. The timing difference can be determinedbased on the measurements of at least one radio receiver positioned at aknown location. The mobile station moreover measures the difference inthe reception time of signals transmitted by the same pairs of radiotransmitters.

The location of the mobile station can then be calculated based on thetime differences measured at the mobile station and the known timingdifference of the at least two pairs of radio transmitters.

For illustration, an example of such a localization method will now bedescribed in more detail with reference to FIG. 1.

FIG. 1 schematically shows a GSM (Global System for Mobilecommunications) radio communications system with three GSM basetransceiver stations BTS 1–3. The base transceiver stations BTS 1–3constitute a network of radio transmitters. Further, two radio receiversLMU1,LMU2 and a mobile station MS are shown. In GSM Location Servicesstandards, such receiving units are called Location Measurement Units(LMU). In the depicted system, the current location of the mobilestation MS is to be determined.

A network of radio transmitters like the depicted base transceiverstations BTS 1–3 has some timing system, which is based for example onrespective internal clocks or on a clock of an element located higher inthe network hierarchy. Each radio transmitter transmits radio signalsaccording to the clock signal with which it is provided and some set ofrules. The time differences between the used clocks determines how muchearlier or later one radio transmitter sends a signal than a respectiveother radio transmitter.

Since GSM is a TDMA (Time Division Multiple Access) system, the depictedGSM base transceiver stations BTS 1–3 transmit radio bursts in timeslots using basically the same time slot structure. To this end, eachGSM base transceiver stations gets a hierarchical clock signal from thenetwork, e.g. through PCM (Pulse Code Modulation) lines. In theory twobase transceiver stations, for example BTS 1 and BTS 2 of FIG. 1, shouldtherefore send TDMA bursts exactly at the same moment. However, this isnot the normal case. In practice, timing differences still exist. Thetiming difference between two base transceiver stations is also referredto as Real Timing Difference (RTD). If base transceiver station BTS 1sends a burst at the time t₁ and base transceiver station BTS 2 sends aburst at the time t₂, the Real Timing Difference between them isRTD=t₁−t₂ or RTD=t₂−t₁, depending on the definition of the RTD. Only ifthe network is really synchronized, the Real Timing Difference is zero.

A possibility for determining the Real Timing Difference betweendifferent radio transmitters is described e.g. in PCT/EP97/02400. Forthe method proposed in this document, radio receivers have to besituated at fixed positions around the radio transmitters. In FIG. 1,this is given by the measurement units LMU1,2 which are positioned atknown locations close to the base transceiver stations BTS 1–3.

The first measurement unit LMU1 then receives a radio signal from thefirst base transceiver station BTS 1 and from the second basetransceiver station BTS 2, and determines the time interval betweenthese two receptions: The time interval is the so-called Observed TimeDifference (OTD). In case a burst from the first base transceiverstation BTS 1 is received at a time t₃ and a burst from the second basetransceiver station BTS 2 is received at a time t₄, the OTD for thispair of base transceiver stations is OTD=t₄−t₃ or OTD=t₃−t₄.

It is to be noted that in GSM, the measurement units can be similar to amobile station. In fact in GSM, OTD is already specified as a functionfor mobile stations in order to support the so called pseudo-synchronoushandover. OTD is in this case the time interval, as observed by themobile station, between the reception of two bursts from two differentbase transceiver stations. For OTD measurements in mobile stations, inpractice one base transceiver stations is the serving base transceiverstation, and the other one is one of the neighbor base transceiverstation.

The OTD determined at measurement unit LMU1 is composed of the RTDbetween the first base transceiver station BTS 1 and the second basetransceiver station BTS 2, and of a Geometrical Time Difference GTD:OTD=RTD+GTD  (1)

More specifically, the Geometrical Time Difference is the timedifference in transmission between two signals from two differenttransmitters (BTS) to a single receiver (LMU,MS) due to geometry. If thedistance of the first base transceiver station BTS 1 to the firstmeasurement unit LMU1 is d₁ and the distance of the second basetransceiver station BTS 2 to the first measurement unit LMU1 is d₂, asindicated in FIG. 1, then the Geometrical Time Difference isGTD=(d₁−d₂)/c, where c is the speed of radio waves. Thus, since thepositions of the base transceiver stations BTS 1,2 and the firstmeasurement unit LMU1 are known, GTD values can be calculated, OTDvalues be measured, and RTD values be obtained.

The same measurements are carried out by the second measurement unitLMU2 for obtaining the timing difference RTD of base transceiverstations BTS 1 and BTS 3. In this case, the GTD is calculated based on adistance of the first base transceiver station BTS 1 to the secondmeasurement unit LMU2 of d₃, and on a distance of the third basetransceiver station BTS 3 to the second measurement unit LMU2 of d₄, asindicated in FIG. 1. Alternatively, also the RTD between basetransceiver stations BTS 1 and 3 could have been determined based onmeasurements by the first measurement unit LMU1, in case thismeasurement unit is also located close enough to base transceiverstations BTS 3 for receiving signals.

The thus known Real Timing Difference between at least two pairs of basetransceiver stations BTS 1,2 and BTS 1,3 can then be exploited fordetermining the geographical location of the mobile station MS.

An example for an E-OTD (Enhanced OTD) location method for GSM has beenstandardized in GSM 03.71 “Location Services (LCS) Stage 2”, whichcontains also a general description of E-OTD. The E-OTD location methoddescribed in the standard is based as well on the above mentionedequation (1), only in this case, the RTD is known, while the GTD is tobe determined.

If the GSM mobile station MS performs OTD measurements for signalsreceived from pairs of base transceiver stations BTS 1,2 and BTS 1,3respectively, and corresponding RTD values are known for these pairs,e.g. measured by special RTD receivers as described above, then theGeometrical Time Difference can be calculated for the mobile station foreach pair of base transceiver stations BTS 1,2 and BTS 1,3 based on theabove mentioned equation (1). The respective Geometrical Time Differenceand the information of the position of the corresponding pair of basetransceiver stations BTS 1,2, BTS 1,3 define a hyperbola H1, H2 on whichthe mobile station is located. When at least two hyperbolas H1, H2 aredefined, the estimate of the location of the mobile can be found attheir intersection as shown in FIG. 1.

One possible location system is such that mobile stations perform OTDmeasurements, and report the results to the network, where locationcalculation is done based on the known RTD.

If receivers, like Local Measurement Units, have absolute timeavailable, they can report Absolute Time (AT) values and/or AbsoluteTime Difference (ATD) values as reception times of received signals tothe network. Absolute time can be made available to receivers e.g. basedon the Global Positioning System (GPS). The AT value is a time stampbased on the absolute time clock for the reception of the signal from atransmitter like a base transceiver station. ATD is the same as RTD withthe exception that also at least one AT of the received signals of thepair of transmitters is known.

A problem connected generally to RTD and AT/ATD measurements are NonLine Of Sight (NLOS) conditions. If there is a NLOS situation between aradio transmitter and a radio receiver, the signal is reflected and thusits propagation path contains extra length. The estimate of thegeographic influence on the received signals based on known transmitterand receiver coordinates, e.g. using GTD, is consequently wrong, and theAT/ATD or RTD estimate includes an extra error. Similar problems areencountered with multipath conditions, i.e. when a Line of Sight (LOS)component is present between transmitter and receiver, but also strongreflected components, which might make a detection of the LOS componentdifficult.

SUMMARY OF THE INVENTION

It is an object of the invention to enhance the accuracy and reliabilityof estimates of transmission times of signals transmitted by radiotransmitters. It is equally an object of the invention to enhance theaccuracy and reliability of estimates of changes in the transmissiontimes of signals transmitted by radio transmitters.

For a first aspect of the invention, a method is proposed fordetermining times of transmission of signals transmitted by at least oneradio transmitter. The method comprises as a first step transmittingsignals by the at least one radio transmitter. In a next step,transmitted signals are received by at least two radio receivers. Foreach of the radio receivers, the respective times of reception of thereceived signals are then determined. Further, for each of said radioreceivers the times of transmission of said received signals areestimated. The estimation is based on the determined times of receptionof the received signals and on the geographical location of the at leastone radio transmitter and the respective radio receiver. Finally, theestimated times of transmission are filtered based on a determineddifference in the time levels of the times of transmission estimatedseparately for the at least two radio receivers for obtaining accuratevalues for the transmission times of signals transmitted by said radiotransmitter.

For the first aspect of the invention, moreover a corresponding radiocommunications system is proposed. The radio communications system thuscomprises at least one radio transmitter for transmitting radio signalsand at least two radio receivers for receiving signals transmitted bysaid at least one radio transmitter and for determining the respectivetimes of reception of received signals. The system further comprisesprocessing means. These processing means are suited on the one hand forestimating for each of said radio receivers the times of transmission ofreceived signals based on the determined times of reception of saidreceived signals and on the geographical location of the at least oneradio transmitter and the respective radio receiver. On the other hand,the processing means are suited for filtering the estimated times oftransmission based on a determined difference in the time level of thetimes of transmission estimated separately for the at least two radioreceivers for obtaining accurate values for the transmission times ofsignals transmitted by said radio transmitter.

For a second aspect of the invention, a method is proposed fordetermining changes in the times of transmission of signals transmittedby at least one radio transmitter. This method comprises again as afirst step transmitting signals by the at least one radio transmitterand as a second step receiving transmitted signals by at least two radioreceivers. Then, for each of the radio receivers values indicative ofthe respective changes in the times of reception of the received signalsare determined. These values can represent in particular the changes inthe times of reception or the changes in times of transmission of thereceived signals, which times of transmission are estimated based on thetimes of reception of the received signals. In both cases, the valuesare the same. Finally, the values indicative of the respective changesof the times of reception determined for said at least two radioreceivers are averaged in order to obtain a reliable information aboutthe changes of the times of transmission of the signals transmitted bysaid at least one radio transmitter.

Also for the second aspect of the invention, a corresponding radiocommunications system is proposed. As the system for the first aspect ofthe invention, the radio communications system for the second aspectcomprises at least one radio transmitter for transmitting radio signals,and at least two radio receivers for receiving signals transmitted bythe at least one radio transmitter. Moreover, the system comprisesprocessing means for determining for each of said radio receivers valuesindicative of the respective changes in the times of reception of thereceived signals. The processing means are additionally employed foraveraging the values indicative of the respective changes of thedetermined times of reception determined for said at least two radioreceivers in order to obtain a reliable information about the changes ofthe times of transmission of the signals transmitted by said at leastone radio transmitter.

The invention proceeds from the idea that signals received by severalradio receivers but originating from the same radio transmitter can beused in an advantageous way when determining information about thetransmission times of the signals, if it is taken into account thatdifferent delays on the transmission paths contribute to a difference inthe time level of transmission times of received signals estimated foreach of the different radio receivers. The term delay is used toindicate the portion of the transmission time of received signals thatsurpasses the theoretical transmission time on a LOS transmission pathbetween the transmitter and the respective receiver. The expression timelevel of estimated transmission times is used for referring to thecontinuously increasing level of succeeding values of transmission timesestimated for each of the receivers for the same transmitted signals bytaking into account the respective geographical situation, i.e. inparticular by assuming a LOS transmission path. A lower time level ofone receiver thus indicates that signals transmitted at the same timereach this receiver with less delay than they reach another receiverwith a higher time level. The offset between the time levels of tworeceivers can be assumed to remain essentially constant as long as thegeographical location of the at least one transmitter and the tworeceivers is not changed.

In the first aspect of the invention, the different time levels aretaken into account in a filtering applied to the estimated transmissiontimes for determining accurate time values for the transmission timesproceeding from the received signals. Since the transmission times aredetermined taking into account the geographic location of the radiotransmitter and the radio receivers, basically only the additionaldelays caused by NLOS or multipath conditions are evaluated for thefiltering.

In the second aspect of the invention, the difference in the time levelsdoes not necessarily have to be determined. It is rather taken intoaccount by estimating parameters not affected by an offset between thetime levels, i.e. the drift of the reception times of signals receivedat the at least two receivers. These drifts can simply be averaged inorder to obtain a more accurate information.

Preferably, the employed radio receivers operate on absolute time AT.The invention can then be employed for determining absolute transmissiontimes and therefore also for ATD measurements. In existing realnetworks, AT capable radio receivers, e.g. GSM Local Measurement Units,already have overlapping coverage areas. This means that several radioreceivers receive signals from the same radio transmitters, e.g. GSMbase transceiver stations. Therefore, these existing radio receivers canbe utilized for the invention. In a GSM radio communications system,moreover modified existing Serving Mobile Location Centers (SMLC) can beemployed as processing means of the invention.

In a preferred embodiment of the first aspect of the invention,information from both radio receivers are included in the obtainedaccurate values for the transmission times. To this end, first the timelevels of the estimated transmission times of the receivers arecompared. Then, the time level of a first one of the receivers which isdetermined to be higher than the time level of the estimatedtransmission times of another one of the receivers is adjusted tocorrespond to the lower time level. That means, the estimated times oftransmission are filtered by projecting the times of transmissionestimated based on signals received by a first one of said radioreceivers, which estimated times of transmission are later in time thanthe respective times of transmission estimated based on the signalsreceived by another one of said radio receivers, to the time level ofthe estimated transmission times based on the signals received by saidother radio receiver. As a result, the delay of signals received by thefirst radio receiver is reduced to the delay of the signals received bythe other radio receiver, and the transmission times estimated for bothreceivers can be regarded as accurate values. In case more than tworadio receivers are utilized, the receiver with the lowest time level isdetermined and the times of transmissions estimated for all or selectedones of the other receivers are projected to the time level of the radioreceiver with the lowest time level.

With this embodiment, the quality of determined AT or ATD values will beimproved, since the measurement results for signals with more delay thanother signals are corrected. This embodiment of the invention ismoreover particularly robust.

In another preferred embodiment of the first aspect of the invention,the accurate values for the transmission times are determined based onlythe signals received by the radio receiver that can be assumed toreceive the signals with less delay. To this end, the estimated times oftransmission are filtered by using exclusively the times of transmissionestimated based on the signals received by a first one of said radioreceivers, which estimated times of transmission indicate respectiveearlier times of transmission of said signals transmitted by the firstradio transmitter than the estimated times of transmission based on thesignals received by at least one other radio receiver.

Also with this embodiment of the invention, a better quality ofdetermined AT and ATD values will be achieved, since the worsemeasurement results, i.e. the measurement results of a receiverreceiving signals with more delay, are discarded.

A further improvement of the second embodiment is proposed for the casethat no received signal is available for some period of time at thefirst radio receiver providing the better measurement results. Duringthese periods of time, the times of transmission estimated based onsignals received by another radio receiver providing worse measurementresults are used. Before being used as optimized transmission times,however, these worse estimated transmission times are first corrected bycompensating for a determined offset of the time level of theseestimated transmission times compared to the time level of thetransmission times estimated based on signals received by the firstradio receiver. The offset has to be determined before the time periodin which no received signals are available at the first radio receiver.

With this supplement, the accuracy of the determined final values forthe transmission times will be further improved, since poor quality ATand ATD values can be used to substitute good quality measurements incase of measurement breaks in these good quality values.

In both preferred embodiments of the first aspect of the invention,before filtering the estimated transmission times, the respectiveestimated transmission times can be corrected according to a knowledgeabout expected delays on the respective transmission paths, if suchknowledge is available. This can be in particular a knowledge about theenvironment, which enables an estimation of an expected delay caused byNLOS conditions.

In the second aspect of the invention, the values indicative of therespective changes of times of reception can be averaged with adifferent weighting for the at least two receivers. The weighting can bemade dependent, for example, on an offset between the time levels oftransmission times estimated from the reception times at the differentradio receivers. Thus, the signals of radio receivers with a lower timelevel, indicating a lower delay on the transmission path, can have ahigher influence on the resulting time slopes than the signals receivedby radio receivers with a higher time level.

The second aspect of the invention can be employed in particulartogether with some method for determining the transmission times ofreceived signals.

In both aspects of the invention it is possible that the radio receiversbase their measurements on different signal bursts transmitted by radiotransmitters. In case the radio transmitter transmits bursts to be usedfor the time measurements in predetermined distances in time, and incase one of said radio receivers receives a burst which is at least oncethe predetermined distance in time later than the last burst received byanother one of the radio receivers, it can be assumed that the radioreceivers base their measurements on different bursts. This can becompensated by reducing the difference in the time of reception or thecorresponding estimated time of transmission by a multiple of thepredetermined distance in time to less than the predetermined distancein time, e.g. by a modulo operation, before further processing.

Both aspects of the invention can be employed for a situation, in whicheach radio receiver exclusively receives signals from another one of atleast two radio transmitters, which two radio transmitters have an equaltiming for transmitting signals.

Equally both aspects of the invention can be employed for a situation,in which each radio receiver receives signals from at least twodifferent radio transmitters. Each radio receiver can then determine inaddition to the time of reception of signals from at least one of saidat least two radio transmitters the time difference between signalsreceived from at least two of said at least two radio transmitters. Thedifference between the times of reception can then be used together withan information about the geometrical situation to determine thedifference in timing used by the two radio transmitters for transmittingsignals. This can be done for example as explained above with referenceto equation (1).

The invention can be used in particular for determining a currentlocation of a mobile station. This can be achieved for example by firstdetermining a difference in timing for at least two different pairs ofat least three radio transmitters as mentioned above. A mobile stationcan further determine the difference in the time of reception of signalstransmitted by respectively one of said at least two different pairs ofsaid at least three radio transmitters. The current geographicallocation of the mobile station can then be determined based on therespective differences in the time of reception of signals determined inthe mobile station and on the determined differences in the timing usedby said two pairs of said at least three radio transmitters. This can beachieved for example again as described above with reference to thespecification GSM 03.71.

Both aspects of the invention can be employed in particular in GSM radiocommunications systems, but it can equally be employed in any othersuitable radio transmitter network, like a WCDMA network.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the invention is explained in more detail withreference to drawings, of which

FIG. 1 illustrates a utilization of determined transmission times in aradio communications system for locating a mobile station;

FIG. 2 illustrates a first embodiment of the invention;

FIG. 3 is a diagram further illustrating the first embodiment of theinvention; and

FIG. 4 is a diagram illustrating a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the first aspect of the invention is describedwith reference to FIGS. 2 and 3.

The upper part of FIG. 2 schematically shows a part of a GSM radiocommunications system. The depicted part of the system comprises a basetransceiver station BTS and two local measurement units LMU A, B. Bothmeasurement units LMU A, B operate based on absolute time which isobtained by the GPS. In this system, the absolute time of transmissionof signals transmitted by the base station are to be determined.

Signals transmitted by the base transceiver station BTS are received bythe measurement units LMU A, B. Respective theoretical LOS paths betweenthe base transceiver station BTS and the measurement units LMU A, B areindicated in the figure by dotted lines. In practice; however, thesignals will propagate between the base transceiver station BTS and thetwo measurement units LMU A, B along paths that may result in additionaldelays in the measurement due to NLOS or multipath conditions. Solidlines depicted in FIG. 2 in addition to the LOS paths between the basetransceiver station BTS and the measurement units LMU A, B indicate thereal NLOS paths taken in this example by signals transmitted by the basetransceiver station BTS.

Each of the measurement units LMU A, B determines the absolute time ofreception of received signals. The determined AT reception values arethen forwarded by the measurement units LMU A, B to a Serving MobileLocation Center SMLC, which is not depicted in FIG. 2, for furtherprocessing.

The measurements of the measurement units LMU A, B are not coordinated,i.e. they may base their measurements on different bursts. In GSM,signals are usually transmitted according to a time slot structure, morespecifically every 0.577 ms. Thus the time difference betweentransmissions of bursts is a multiple of 0.577 ms. If the measurementunits LMU A, B receive signals with a time difference of more than 0.577ms, then the measurements have most probably been made based ondifferent transmitted bursts. The measurement results can therefore bemade comparable in the SMLC by reducing the time difference with amodulo 0.577 ms division to less than 0.577 ms. Then the receivedsignals can be assumed to result from the same transmitted burst.

The SMLC knows the geographical location of the two measurement unitsLMU A, B and of the base transceiver station BTS, and thus the assumedLOS distances d5, d6 between the base station and the respectivemeasurement unit LMU A, B. Analogously to the example described withreference to FIG. 1 and equation (1), the known LOS delay is thus d5/cfor signals received at the first measurement unit LMU A and d6/c forsignals received at the second measurement unit LMU B. By subtractingthe respective LOS delay from the respective determined AT receptionvalues, the SMLC can thus estimate corresponding AT transmission valuesfor each measurement unit LMU A, B, which AT transmission valuesconstitute two separate preliminary estimations of the real transmissiontimes.

If the absolute time values are calculated from the measurements of bothmeasurement units LMU A, B as described above, the AT transmissionvalues estimated for the first measurement unit LMU A will be smallerthan the AT transmission values estimated for the second measurementunit LMU B, even though the burst on which the measurement was based isor can be considered the same. The reason for this is that for thesecond measurement unit LMU B, the real NLOS path adds more additionaltransmission path to the theoretical LOS path than for the firstmeasurement unit LMU A. This difference is also depicted in the lowerpart of FIG. 2, which shows a time line in terms of time slots. On thistime line, the AT transmission value estimated for one burst for eachmeasurement unit LMU A, B is associated to a specific time slot of thetime slot structure used by the base station.

In the first embodiment of the invention, the SMLC thus assumes thatsignals of the same bursts which result in a lower estimated ATtransmission value have traveled a more direct path, since NLOS andmultipath conditions always cause additional delays. As consequence, theSMLC only uses the respective lower estimated AT transmission valueswhen determining the final AT transmission value. These lower ATtransmission values are based in the example of FIG. 2 on themeasurement results of the first measurement unit LMU A.

Only in case there is a break in the measurements of the firstmeasurement unit LMU A for some reason, the SMLC will use themeasurement results of the second measurement unit LMU B.

FIG. 3 illustrates how this can be carried out. The figure is a diagramdepicting the estimated AT transmission values for signals transmittedby the base station over time. An upper curve in a dotted linecorresponds to the AT transmission values determined for the secondmeasurement unit LMU B, while a lower curve in a solid line correspondsto the AT transmission values determined for the first measurement unitLMU A. The lower curve, however, is disrupted due to a break in themeasurement of the first measurement unit LMU A.

During this disruption, the SMLC utilizes the AT transmission valuesestimated according to the measurement results of the second measurementunit LMU B. The AT transmission values estimated for the secondmeasurement unit LMU B are corrected before being used with the offsetbetween the second and the first measurement unit LMU B, A, which offsetwas determined in the SMLC before the break occurred. At the break ofthe solid line in the lower curve, the corrected AT transmission valuesare depicted as dotted line. The result is a basically continuous curveobtained for the final AT transmission values.

A second embodiment of the first aspect of the invention is equallybased on a GSM radio communications system with a base transceiverstation BTS and two measurement units LMU A, B as depicted in the upperpart of FIG. 2. Also in this second embodiment the absolute times oftransmission of signals transmitted by the base transceiver station BTSare to be determined.

Signals are transmitted by the base transceiver station BTS and receivedby the two measurement units LMU A, B as described with reference to thefirst embodiment. Moreover, AT reception values are determined in thetwo measurement units LMU A, B as described with reference to the firstembodiment, and preliminary AT transmission values are estimated againin an SMLC based on the AT reception values determined in the first andthe second measurement unit LMU A, B.

Only the processing of the preliminary AT transmission values isdifferent in the second embodiment of the invention, as will beexplained with reference to FIG. 4.

FIG. 4 is a diagram depicting preliminary AT transmission valuesestimated for the first and the second measurement unit LMU A, B overtime. The AT transmission values of the first measurement unit LMU A aredepicted as white circles, and the AT transmission values of the secondmeasurement unit LMU B are depicted as black circles. The ATtransmission values of the second measurement unit LMU B are positionedat a higher time level than those of the first measurement unit LMU A,i.e. they suggest a later time of transmission because of a longer delayby an NLOS transmission.

In this embodiment, however, the SMLC does not discard completely the ATtransmission values which are based on the measurements of themeasurement unit LMU B with higher AT transmission values, but ratheruses the AT transmission values based on the measurement results of bothmeasurement unit LMU A, B.

To this end, first the AT transmission values determined by bothmeasurement units LMU A, B are filtered separately using a linear model,preferably a linear Kalman filter. Therefore, the black and whitecircles of FIG. 4 are connected by lines indicating the respectiveexpected continuation of AT transmission values after each estimated ATtransmission value.

Thus, a current offset between the AT transmission values of the twomeasurement units LMU A, B can be extrapolated each time a new ATtransmission value is estimated for the second measurement unit LMU B.The AT transmission values estimated for the second measurement unit LMUB are then projected to the time level of the AT transmission valuesestimated for the first measurement unit LMU A. The projection isindicated in FIG. 4 by arrows, and the resulting projected ATtransmission values are depicted as gray circles.

In addition, in case there is some extra knowledge about expected delayscaused by NLOS conditions, for example a common knowledge about theenvironment, the AT reception or transmission values might be correctedadditionally before the AT transmission values are filtered.

In both embodiments of the first aspect of the invention, the determinedAT transmission values can be used in particular for determining moreaccurate ATD values for two different base transceiver stations and thusfor locating more accurately a mobile station as described withreference to FIG. 1.

In an embodiment of the second aspect of the invention, which is basedagain on the GSM communications system depicted in the upper part ofFIG. 2, the changes in the transmission times of the base transceiverstation BTS are to be determined reliably for further use in the GSMnetwork.

For this embodiment of the invention it is assumed that the effect ofNLOS and mulipath conditions is simply an extra, time-independent delay.Therefore, the time behavior, or drift, of the AT reception valuesshould be the same for both measurement units LMU A, B, since a constantoffset should not affect speed and direction of the drift.

Consequently, the reliability of the measurements of a singlemeasurement unit can be improved by averaging the changes in the ATreception or transmission values for different measurement units LMU A,B.

The changes in the AT reception or transmission values can in particularbe averaged with a different weighting for different measurement unitsLMU A, B. The measurement unit LMU A with a lower time level can thenhave a greater influence on the resulting slopes than the measurementunit LMU B with a higher time level.

1. Method for determining times of transmission of signals transmittedby at least one radio transmitter (BTS), the method comprising:transmitting signals by said at least one radio transmitter (BTS);receiving transmitted signals by at least two radio receivers (LMU A,B);determining for each of said radio receivers (LMU A,B) the respectivetimes of reception of the received signals; estimating for each of saidradio receivers (LMU A,B) times of transmission of said received signalsbased on the determined times of reception of said received signals andon the geographical location of the at least one radio transmitter (BTS)and the respective radio receiver (LMU A,B); filtering the estimatedtimes of transmission based on a determined difference in the timelevels of the times of transmission estimated separately for the atleast two radio receivers (LMU A,B) for obtaining accurate values forthe transmission times of signals transmitted by said radio transmitter(BTS).
 2. Method according to claim 1, wherein the estimated times oftransmission are filtered by projecting the times of transmissionestimated based on signals received by a first one of said radioreceivers (LMU B), which estimated times of transmission are later intime than the respective times of transmission estimated based on thesignals received by another one of said radio receivers (LMU A), to thetime level of the transmission times estimated based on the signalsreceived by said other radio receiver (LMU A).
 3. Method according toclaim 1, wherein the estimated times of transmission are filtered byusing exclusively the times of transmission estimated based on thesignals received by a first one of said radio receivers (LMU A) asaccurate values, which estimated times of transmission indicaterespective earlier times of transmission of signals transmitted by saidfirst radio transmitter (LMU A) than the times of transmission estimatedbased on the signals received by the at least one other radio receiver(LMU B).
 4. Method according to claim 3, wherein in case no receivedsignal is available for some period of time at said first radio receiver(LMU A), the times of transmission estimated based on signals receivedby one of said at least one other radio receiver (LMU B) are used duringsaid period of time as accurate values after compensating an offset ofthe time level of the transmission times estimated based on signalsreceived by said at least one other radio receiver (LMU B) compared tothe time level of the transmission times estimated based on signalsreceived by said first radio receiver (LMU A), which offset isdetermined before said period of time.
 5. Method according to claim 1,wherein before filtering the estimated transmission times, thedetermined reception times or the estimated transmission times arecorrected according to a knowledge about expected delays on therespective transmission paths.
 6. Method according to claim 1, whereinsignals are transmitted by said at least one radio transmitter (BTS) inmultiples of a predetermined distance in time, and wherein in case oneof said radio receivers (LMU A,B) receives signals which are at leastonce the predetermined distance in time later than the last signalsreceived by another one of the radio receivers (LMU B,A), the differencebetween the determined times of reception of the at least two radioreceivers (LMU A,B) or between the corresponding estimated times oftransmission is reduced by a multiple of the predetermined distance intime to a difference below the predetermined distance in time beforefurther processing.
 7. Method according to claim 1, wherein each radioreceiver receives signals from another one of at least two radiotransmitters, which at least two radio transmitters have an essentiallyequal timing for transmitting signals, and wherein a common accuratetransmission time is obtained for the at least two radio receivers byfiltering the estimated transmission times.
 8. Method according to claim1, wherein each radio receiver receives signals from at least twodifferent radio transmitters for at least one of which transmittersaccurate values for the times of transmission are determined byfiltering estimated transmission times, wherein in addition thedifference in the reception time between signals received from said atleast two radio transmitters is determined for at least one radioreceiver, and wherein based on the determined difference in thereception time and on information about the geometrical situation thedifference in timing used by the two radio transmitters for transmittingsignals is determined.
 9. Method according to claim 8, wherein adetermined difference in timing used by at least two radio transmittersfor transmitting signals is exploited for determining a current locationof a mobile station receiving signals from said at least two radiotransmitters.
 10. Method according to claim 8, wherein a difference intiming for transmitting signals is determined for at least two differentpairs of at least three radio transmitters, wherein for each of the atleast two pairs of radio transmitters the difference in the time ofreception at a mobile station of signals transmitted by the radiotransmitters of the respective pair of radio transmitters is determined,and wherein the current geographical location of the mobile station isdetermined based on the respective differences in the time of receptionof signals determined in the mobile station and on the determineddifferences in the timing used by said two pairs of radio transmitters.11. Method for determining changes in the times of transmission ofsignals transmitted by at least one radio transmitter (BTS), the methodcomprising: transmitting signals by said at least one radio transmitter(BTS); receiving transmitted signals by at least two radio receivers(LMU A,B); determining for each of said radio receivers (LMU A,B) valuesindicative of the respective changes in the times of reception of thereceived signals; averaging the values indicative of the respectivechanges of times of reception determined for said at least two radioreceivers (LMU A,B) for obtaining a reliable information about thechanges of the times of transmission of the signals transmitted by saidat least one radio transmitter (BTS).
 12. Method according to claim 11,wherein signals are transmitted by said at least one radio transmitter(BTS) in multiples of a predetermined distance in time, and wherein incase one of said radio receivers (LMU A,B) receives signals which are atleast once the predetermined distance in time later than the lastsignals received by another one of the radio receivers (LMU B,A), thedifference between the determined times of reception of the at least tworadio receivers (LMU A,B) or between the corresponding estimated timesof transmission is reduced by a multiple of the predetermined distancein time to a difference below the predetermined distance in time beforefurther processing.
 13. Method according to claim 11, wherein each radioreceiver receives signals from another one of at least two radiotransmitters, which at least two radio transmitters have an essentiallyequal timing for transmitting signals, and wherein a common accuratetransmission time is obtained for the at least two radio receivers byfiltering the estimated transmission times.
 14. Method according toclaim 11, wherein each radio receiver receives signals from at least twodifferent radio transmitters for at least one of which transmittersaccurate values for the times of transmission are determined byfiltering estimated transmission times, wherein in addition thedifference in the reception time between signals received from said atleast two radio transmitters is determined for at least one radioreceiver, and wherein based on the determined difference in thereception time and on information about the geometrical situation thedifference in timing used by the two radio transmitters for transmittingsignals is determined.
 15. Radio communications system comprising atleast one radio transmitter (BTS) for transmitting radio signals, atleast two radio receivers (LMU A,B) for receiving signals transmitted bysaid at least one radio transmitter (BTS) and for determining therespective times of reception of received signals; processing means forestimating for each of said radio receivers (LMU A,B) times oftransmission of said received signals based on the determined times ofreception of said received signals and on the geographical location ofthe at least one radio transmitter (BTS) and the respective radioreceiver (LMU A,B), and for filtering the times of transmissionestimated based on a determined difference in the time level of thetimes of transmission estimated separately for the at least two radioreceivers (LMU A,B) for obtaining accurate values for the transmissiontimes of signals transmitted by said radio transmitter (BTS).
 16. Radiocommunications system according to claim 15, wherein said processingmeans filter the estimated times of transmission by projecting the timesof transmission estimated based on signals received by a first one ofsaid radio receivers (LMU B), which estimated times of transmission arelater in time than the respective times of transmission estimated basedon the signals received by another one of said radio receivers (LMU A),to the time level of the transmission times estimated based on thesignals received by said other radio receiver (LMU A).
 17. Radiocommunications system according to claim 12, wherein said processingmeans filter the estimated times of transmission by using exclusivelythe times of transmission estimated based on the signals received by afirst one of said radio receivers (LMU A) as accurate values for thetimes of transmission, which estimated times of transmission indicaterespective earlier times of transmission of said signals transmitted bysaid radio transmitter (BTS) than the times of transmission estimatedbased on the signals received by at least one other of said radioreceivers (LMU B).
 18. Radio communications system according to claim17, wherein in case there is no received signal available for someperiod of time at said first radio receiver (LMU A), said processingmeans use during said period of time the times of transmission estimatedbased on signals received by one of said at least one other radioreceiver (LMU B) as accurate values after compensating an offset in thetime level of the transmission times estimated based on signals receivedby said other radio receiver (LMU B) compared to the time level of thetransmission times estimated based on signals received by said firstradio receiver (LMU A), which offset is determined before said period oftime.
 19. Radio communications system according to claim 15, wherein theprocessing means correct the determined reception times or the estimatedtransmission times according to a knowledge about expected delays on therespective transmission paths before filtering the estimatedtransmission times.
 20. Radio communications system according to claim15, wherein signals are transmitted by said at least one radiotransmitter (BTS) in multiples of a predetermined distance in time, andwherein, in case one of the radio receivers receives (LMU A) a signalwhich is at least once the predetermined distance in time later than thelast signal received by another one of said radio receivers (LMU B), theprocessing unit reduces the difference between the determined times ofreception of the at least two radio receivers (LMU A,B) or between thecorresponding estimated times of transmission by a multiple of thepredetermined distance in time to a difference below the predetermineddistance in time before further processing.
 21. Radio communicationssystem according to claim 15, wherein the at least one radio transmitteris at least two radio transmitters which have an essentially equaltiming for transmitting signals, wherein each radio receiver receivessignals from another one of said at least two radio transmitters, andwherein the processing means obtain a common accurate transmission timefor the at least two radio receivers by filtering the estimatedtransmission times.
 22. Radio communications system according to claim15, wherein said at least one radio transmitter is at least two radiotransmitters, wherein each of said at least two radio receivers receivessignals from at least two different radio transmitters, the processingmeans determining for at least one of said radio transmitters accuratevalues for the times of transmission by filtering estimated times oftransmission, wherein the processing means determine in addition thedifference in the reception time of signals received from said at leasttwo radio transmitters for at least one radio receiver, and wherein theprocessing means determine based on the determined difference in thereception time and on information about the geometrical situation thedifference in timing used by the two radio transmitters for transmittingsignals.
 23. Radio communications system according to claim 22 furtherincluding a mobile station, wherein said at least two radio transmittersare at least three radio transmitters, wherein said mobile station orsaid processing means determines for each of the at least two pairs ofradio transmitters the difference in the time of reception of signalstransmitted by the radio transmitters of the respective pair of radiotransmitters, and wherein the processing means determine the currentgeographical location of said mobile station based on the respectivedifferences in the time of reception of signals determined in the mobilestation and on the determined differences in the timing used by said twopairs of radio transmitters for transmitting signals.
 24. Processingmeans for a radio communications system according to claim
 15. 25. Radiocommunications system comprising at least one radio transmitter (BTS)for transmitting radio signals, at least two radio receivers (LMU A,B)for receiving signals transmitted by the at least one radio transmitter(BTS); and processing means for determining for each of said radioreceivers (LMU A,B) values indicative of the respective changes in thetimes of reception of the received signals, and for averaging the valuesindicative of the respective changes of the determined times ofreception determined for said at least two radio receivers (LMU A,B) inorder to obtain a reliable information about the changes of the times oftransmission of the signals transmitted by said at least one radiotransmitter (BTS).
 26. Radio communications system according to claim25, wherein signals are transmitted by said at least one radiotransmitter (BTS) in multiples of a predetermined distance in time, andwherein, in case one of the radio receivers receives (LMU A) a signalwhich is at least once the predetermined distance in time later than thelast signal received by another one of said radio receivers (LMU B), theprocessing unit reduces the difference between the determined times ofreception of the at least two radio receivers (LMU A,B) or between thecorresponding estimated times of transmission by a multiple of thepredetermined distance in time to a difference below the predetermineddistance in time before further processing.
 27. Radio communicationssystem according to claim 25, wherein the at least one radio transmitteris at least two radio transmitters which have an essentially equaltiming for transmitting signals, wherein each radio receiver receivessignals from another one of said at least two radio transmitters, andwherein the processing means obtain a common accurate transmission timefor the at least two radio receivers by filtering the estimatedtransmission times.
 28. Radio communications system according to claim25, wherein said at least one radio transmitter is at least two radiotransmitters, wherein each of said at least two radio receivers receivessignals from at least two different radio transmitters, the processingmeans determining for at least one of said radio transmitters accuratevalues for the times of transmission by filtering estimated times oftransmission, wherein the processing means determine in addition thedifference in the reception time of signals received from said at leasttwo radio transmitters for at least one radio receiver, and wherein theprocessing means determine based on the determined difference in thereception time and on information about the geometrical situation thedifference in timing used by the two radio transmitters for transmittingsignals.
 29. Processing means for a radio communications systemaccording to claim 25.